84th AMS Annual Meeting

Wednesday, 14 January 2004
The role of the helicopter-borne turbulence probe Helipod in joint field campaigns
Hall AB
Thomas Spiess, Aerospace Systems/Technical Univ., Braunschweig, Germany; and P. Zittel and J. Bange
Poster PDF (1.6 MB)
Increasing efforts in numerical climate and weather prediction require precise knowledge of atmospheric exchange processes. In the early 1990s the German BMBF funded the development and construction of a new airborne turbulence probe named Helipod. The requirement on this in-situ system was to increase the accuracy of near-surface turbulent flux measurements by about one order of magnitude. Since 1994 the Helipod participated several field campaigns over land and in the Arctic.

To date the Helipod is a unique measurement system worldwide. It is attached to a 15 m rope under a helicopter of nearly any type and can operate autonomously since it has its own power supply, navigation, on-line data processing and storage. The disturbance of the atmospheric current by the system is minimal, since its fuselage is comparative small (5 m long and 60 cm diameter) and it does not need wings or propulsion. During the measurement flight the Helipod is out of the downwash area of the helicopter. Due to its sampling frequency of 100 Hz at a mean true air speed of 40 m/s and its fast sensors the spatial resolution is in the sub-meter range. It is equipped with several sensors to measure the atmospheric wind vector, humidity, air and surface temperature over a wide spectral range. This concept enables the Helipod to resolve small scale turbulence and turbulent transport of momentum, heat, and moisture.

With a few exceptions the Helipod operated always in joint field campaigns together with ground-based stations, remote sensing systems, and numerical models. The Helipod measurements were often taken as reference, cross-validation or initialization (especially LES models). In the framework of LITFASS field campaigns the area-averaged turbulent surface fluxes determined from Helipod flights are used to define an averaging method for ground measurements. In the German Antarctica research program, Helipod data are now taken to analyze the small-scale turbulent structure of the ground inversion above sea ice.

On most campaigns the Helipod operated in the ABL up to maximum altitude of 1500 m for horizontal flights. The measurements can be completed by high flying research aircrafts e.g., to observe boundary layer entrainment and turbulent surface fluxes simultaneously. In the combination with wind profilers, LIDAR, and SODAR both horizontal and vertical variations in the atmosphere were observed e.g., in LITFASS.

A specialty of the system are ship-based missions. Since 1995 the Helipod participated several Arctic expeditions aboard e.g. the research vessel Polarstern. The system is completely demountable and can be transported in boxes around the world on a ship or in a commercial aircraft. The re-assembly takes a few hours, while the helicopter can be rented at the operational area e.g. as performed during PHELIX 1997 in California (together with NOAA, NCAR). Missions in the tropical (rainforest) areas are possible as well.

The Helipod is no subject to approval (FAA, JAA) and may in general perform low-level flights in (sparsely) populated regions. The mission range of the Helipod is only limited by the size of data media and the endurance of the helicopter with is typically 3 hours. To date no flights within clouds have been performed since most helicopters and pilots are just allowed to fly according to the visual flight rules (VFR). Compared to turboprop planes the operating costs of the helicopter are somewhat higher (about 1500 EURO or USD).

At this time the Helipod receives an upgrade that will be completed in mid 2004. The main computer will be renewed and the data recording rate will be increased to 500 Hz with an oversampling of several kHz for noise reduction. A renewed GPS- and alignment system with 3 GPS receivers and antennas will be combined with the ARINC interface for the LITEF inertial navigation system to improve high-resolution GPS-based attitude information. The data will be stored on a flash storage with several GByte and processed offline (post processing) for better (complementary or Kalman) filtering, especially of the navigation data. The latter step will be done to make system change more easy and rapid. Additional sensors are planned measuring water droplets, CO_2 and other tracers.

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